Study on the ultrasonic detection method for internal defects of insulation pull rod in circuit breakers

To improve the accuracy and validity of ultrasonic detection for insulation pull rod, a suitable ultrasonic focusing probe and the corresponding analysis system are developed in this paper, considering the material property, structure characteristic and the features of typical internal defects. In addition, a series of samples containing typical defects were tested to acquire the time-domain and frequency-domain characteristics, based on which an effective method that use the echo amplitude of defect and the second peak of spectrum to identify the defects is proposed. Experimental results show that in case of void defects, the time interval between surface echo and defect echo increases with the increase of defect depth. On the contrary, the second peak of spectrum has negative correlation with the depth and size of the void defects. When insulation pull rob is subjected to strong stress, the insulation pull rob appears layered and degummed, which result in the echo of time-domain and frequency-domain was distorted a lot. In conclusion, the combination of the time-domain and frequency-domain characteristics can successfully diagnosis defects in insulation pull rod.